The technology has recently been developed for the production of high-quality thin dielectrics which make possible the commercial utilization of the Fowler-Nordheim electron tunneling mechanism. This process can be used in the operation of electrically erasable programmable read only memory (EEPROM) devices. The electron tunneling permits the storage of a charge on a floating gate for an extended period of time even when operating power has been removed from the memory device.
The programming of a memory cell using the Fowler-Nordheim mechanism for present thin dielectrics requires approximately 20 volts. However, the standard power supply for microprocessor and memory circuits is 5 volts. The required high voltage can be produced by charge pump circuits which are fabricated integrally with the memory device. Although charge pump circuits can produce this required high voltage, these circuits can provide only a very limited current with reasonably sized devices. Thus, it is necessary that selected load elements draw only a small current from the charge pump while nonselected load elements exhibit a high impedance to prevent loading the charge pump.
A particular difficulty in the design of dynamic load circuits to carry out the above-listed requirements is that the application of high voltages to the source terminals of the MOS transistors tends to substantially increase the threshold voltages of the transistors. This is termed "body effect". This increase in threshold voltage must be accounted for so that the required high voltage can be produced for application to the Fowler-Nordheim circuit element. A method for controlling the threshold voltage of the transistors in the dynamic load circuit is to carefully regulate the processing steps in the manufacture of the memory device. This careful regulation of the processing steps can significantly increase the per unit cost of the memory devices.
Therefore, in view of the requirement to have a load control circuit for programming EEPROM circuits, there exists a need for such a circuit which can carry out the functions of driving a circuit node, when selected, to approximately the charge pump voltage, while drawing essentially no current from the charge pump when the circuit node is not selected, but at the same time having a relatively broad range of the threshold voltages for the dynamic load circuit transistors thereby not imposing strict processing controls.